Heat-sealed edge joint and method



Nov. 27, 1962 D. L. POMMER HEAT-SEALED EDGE JOINT AND METHOD I FiledJune 3. 1960 2 Sheets-Sheet l FIG. 4

INVENTOR. DEN/v1.5 L. POMMER ATTORNEY:

Nov. 27, 1962 D. L. POMMER 3,066,064

HEAT-SEALED EDGE JOINT AND METHOD Filed June 3. 1960 2 Sheets-Sheet 2INVENTOR. DENNIS L PoMMER 3,956,054 Patented Nov. 27, 1962 Free3,066,064 HEAT-SEALED EDGE JOINT AND METHOD Dennis L. Pomrner,Minneapolis, Minn., assignor to Bemis Bro. Bag Company, Minneapolis,Minn., a corporation of Missouri Filed June 3, 1950, Ser. No. 33,799 10Claims. (Cl. 156-306) This invention relates to heat-sealed jointconstructions andmore particularly to heat-sealed edge joints forsealing the edges of manufactured articles made from thermoplasticresinous sheet and film and includes the method for making such seals.The term edge joint as used herein includes broadly a joint runningalong aligned superimposed edges of two or more layers of sheet materialand is to be distinguished from other joints such as lap joints, andbutt joints and the like.

In many cases there has been a great deal of difficulty in obtaining astrong heat-sealed joint between two or more layers of plastic film orsheet, and particularly in the case of polyolefin film, such aspolyethylene and polypropylene. A suitable seal is one in which thestrength of the seal is very nearly the same as that of the film itself.While seals of satisfactory strength have often been obtained with lightgage film ranging from 1 to 3 mils, poor seal strength is encounteredespecially when the film is of a fairly heavy gage, such as from 5 to 15mils in thickness.

After conducting a large number of tests wherein two sheets of milpolyethylene film were sealed together using prior art joints of varioustypes I have found that strength of the joints was in no case more than75% of the film strength and in some cases ranged as low as 55% of thefilm strength, even though optimum sealing temperature, pressure andtime were selected in each case. If the temperature is too low duringsealing or the sealing time is too short, the seal will be imperfect andthe joint will be weaker than the film surrounding it. The followingdiscussion therefore assumes that the best temperature, pressure andsealing times are selected.

Previous methods of sealing plastic film relied on fusing twosuperimposed sheets together by heat, with or without pressure, in asingle operation. One disadvantage of this method is that sealed arealoses most of the molecularorientation and resulting strength that wasachieved during extrusion. The object of most film extrusion processesis to orient molecules of the plastic film to bring about the bestphysical properties. In sealing the film, the greater part of theorientation is lost because the film is not subject to tension duringcooling and, as a result,

the material in the seal is weaker than the material of the film itself.This weakness can be demonstrated easily by conventional tensile testingmethods. The second disadvantage of this method is that thethermoplastic material, when it is heated to fusion temperature, cannotsupport the pressure of the heated sealing surfaces without flowingand-consequently thinning down to some extent.

Recently heavy gage polyolefin films have become very important in theindustrial packaging field.v One notable application for such film is inmanufacturing heavy gage polyethylene shipping bags. These bags are airtight, water impervious, and can be stored out of doors and thereforeperform better than most prior bags of conventional paper or textileconstruction, even though the latter may include several layers ofmaterial as well as various moisture barriers and coating. A problemwhich has limited the successful use of these polyethylene containers isthat heretofore no satisfactory seal could be made in bottom and top ofthe bag. As a result, the film thick ness of the bag had to be increasedbeyond that which was actually necessary solely to assure that the topand bottom seals would be sufficiently strong. This, of course,

raised raw material and production costs. I have found, however, thatthrough the use of my improved joint construction, it has been possibleto increase the strength of the top and bottom seal of 10 milpolyethylene shipping sacks very greatly beyond that which waspreviously possible, always over of the film strength.

Broadly stated, my improved heat sealed joint comprises a band ofresinous material formed from the material from which the sealed sheetsare made having a generally bulbous cross-sectional appearance,considerably thicker than the combined thickness of the layers of sealedfilm and having an increasing cross-sectional thickness toward the freeedge thereof.

A preferred method of making the joint consists of first sealingsuperimposed layers of sheet material together with heat and pressurealong a narrow band parallel to and spaced inwardly from the edges ofthe sheets to be sealed, the edges of the sheet material projectingoutwardly from the seal comprising an extension of unsealed sheetmaterial, and thereafter subjecting the seal to heat above the meltingtemperature thereof for a period of time less than that required tobring the entire exposed portions of said sheet material to the meltingtemperature thereof. By this means the extensions are caused to meltinto a head which is the greater thickness of said seal. Thecross-sectional thickness of the seal is increased and the material fromsaid extensions is formed into a bulbous mass on the free edge of saidjoint.

It is one object of this invention to provide an edge joint constructionfor thermoplastic resinous film and sheet which has satisfactorystrength when used with heavy gages as well as light gage sheetmaterial.

It is another object of this invention to provide an edge joint forthermoplastic resinous sheet ranging in thickness up to 15 mils which isat least 95% as strong as the film itself.

It is a further object of this invention to provide an improved heatsealed joint construction for heavy gage plastic bags.

It is a still further object of this invention to provide a rapid andeconomical method for forming strong seals in heavy gage resinous sheetmaterial.

Other objects will become apparent as the description proceeds.

To the accomplishment of the foregoing and related ends, this inventionthen comprises the features hereinafter described and particularlypointed out in the claims, the following description setting forth indetail certain illustrative embodiments of the invention, these being,however, but a few of the various ways in which the principles of thisinvention may be employed.

The invention is illustrated by the drawings in which the same numeralsrefer to corresponding parts and in which:

FIGURE 1 is a greatly enlarged perspective end view of two sheets ofresinous plastic sheet material sealed together showing the first stageof forming seals according to this invention;

FIGURE 1A is a somewhat smaller perspective end view showing the normalrupture path of the sealed joint of FIGURE 1;

FIGURE 2 is a greatly enlarged perspective end view of two sheets ofplastic material sealed together by means of the joint constructionaccording to this invention;

FIGURE 2A is a somewhat smaller perspective end view showing the normalrupture path of the sealed joint of FIGURE 2;

FIGURE 3 is a side elevation, in section, of one form of apparatus whichmay be used to manufacture plastic joints according to this invention;

FIGURE 4 is a somewhat enlarged cross sectional side elevation of thatportion of the sealing apparatus of FIG- URE 3 used to form the secondsealing step in manufacturing seals according to this invention; and

FIGURE 5 is a fragmentary end elevation showing the bite of the feedrolls to the second sealing stage of the apparatus.

Referring now to the drawings, there is shown in F1 URE l a first stageseal joint 16 between two sheets of thermoplastic resinous film 11 and12 made in the first stage of producing heat sealed joints according tothis invention. The seal is comprises a band 13 somewhat thicker thanthe seal sheets formed from the resinous material of sheets 11 and 12running parallel to the edges of the sheets and leaving unsealed bandsor extensions 14 and 15 along the edges. The edges 16 of the band aregenerally rounded and the width of the band 13 is desirably betweenabout and of an inch and prefer ably about inch. The thickness of theband 13 is preferably about 1.5 times the combined thickness of thesheets 11 and 12 and is apparently continuous through the center wherethe two sheets are fused together. Extensions 14 and 15 of unsealed filmprojecting outwardly beyond the seal it are preferably about the samewidth as the width of the band 13.

Seal is made Without holding the film under tension. Thus, the sealedband 13 shrinks to about the width of the heated sealing surfaces.Heated sealing surfaces are used to form seal it and are maintained at atemperature of from 250 F. to 500 F. depending upon the particular filmbeing sealed. A preferred temperature range for 10 mil polyethylene filmis about 285 F. and the preferred sealing time is about 2.5 seconds. Thesealing pressure is preferably about 5 lbs. per square inch of seal areabut this may be changed to suit the specific characteristics of themachine being used. Usually a higher sealing temperature is preferredbecause the productivity of the machine will be increased as a result.

Means are desirably provided for accurately controlling the pressureexerted by the sealing surfaces during sealing, because it can be shownthat increasing the ressure reduces the sealing time required at a giventemperature or for given clamping time and higher clamping pressurerequires a lower operating temperature. If the seal ing time is tooshort, of course, no sealing will take place and if it is excessive thecompression of the film caused by the heated sealing surfaces cannot beregained by shrinkage. If the temperature is too low, either no sealingwill take place or the sealing time will be too long so that productionis reduced. If the temperature is too high, the resin may degrade. Ifthe pressure is too low delamination may take place and if too high theseal may be thinned down excessively and hence weakened.

The final joint construction indicated generally at 17 is illustrated inFIGURE 2. The joint consists of a bead or band 18 formed from. theresinous material making up the sealed sheets 11 and 12 extending alongthe edge thereof and having a generally bulbous cross-sectionalappearance. In the portion of the joint nearest the unsealed sheets 11and 12 is an abrupt increase in crosssectional thickness. This end ofthe seal is a rounded portion 19. The center portion 2t? of the sealbecomes gradually thicker in cross-section toward the free end of thejoint and the edge portion 21 of the joint is generally rounded incross-section and its thickness is preferably at least about two timesthe combined thickness of the sheets being sealed.

As noted hereinbefore, I have found that the prior art heat sealedjoints formed from relatively heavy plastic film and sheet areconsiderably below the strength of the sheet material itself. Thestrength of seal described above and illustrated in FIGURE 1 averagedabout 65% to 75% of the seal strength. The strength of the finishedjoint illustrated in FIGURE 2, on the other hand, is at least 95% of thefilm strength.

By examining the joint as the sheets 11 and 12 were pulled apart slowly,I found that the joint illustrated in FEGURE 1A would split into twosections designated 22 and 23 approximately along dotted line 24.Sections 22 and 23 of the joint on the opposite sides of the seal werefound to stretch considerably as tension was increased, probably as aresult of having become weakened through the loss of the molecularorientation during the sealing process. In almost every test, rupturetook place in section 22 or 23. In some instances, however, thespecimens split all the way through the center approximately where thetwo sheets had been scaled together originally.

In the case of the joint construction according to this inventionillustrated in FIGURE 2A, the same kind of splitting took placeapproximately along dotted line 25 but would never proceed past thecenter of the joint, probably because of the greater thickness of thelarge mass of material 21 at the outer edge of the joint. It was alsofound that sections 26 and 27 did not rupture. Instead, rupture tookplace in either sheet 11 or 12 at a short distance from the joint. Thisis the reason why the strength of the joint according to invention isalways very nearly the same as the strength of the film itself.

Referring now to FIGURES 3, 4 and 5, there is shown one form ofapparatus for producing heat-sealed joints according to this invention.The various parts of the apparatus are supported by a suitable frameincluding a plate 2%. Plastic sheet material, such as tubing 30 which isto be made into bags, is withdrawn from a supply roll 31 by a pair ofcooperating draw rolls 32 and 33 which are driven in a given feeddirection by means not shown, and supported by metal uprights 34 ateither end. The rolls are preferably covered by a coating 35 ofresilient material such a rubber to assure that the film tubing 30 isgripped securely.

From the draw rolls 32 and 33 the tubing passes between a pair of heatedbars 37 and 38. Heat is supplied by conventionalelectric heatingelements 39 recessed within the bars. Each bar has a raised lip portion46 running its length which is adapted to contact the tubing 3% when thebars 37 and 38 are forced together by conventional commercial pneumaticactuator 41. The bars 37 and 33 form a seal joint similar to thatillustrated in FIGURE 1. Downstream from the bars 37 and 38 is mountedhorizontaliy a pneumatic actuator which is used to move a cutting head46 horizontally across the film. Rotatably attached to the cutting headis a steel cutting wheel 47 having a sharp outer edge. Below the cutteris a fiat metal plate :8 against which the cutting wheel 47 acts insevering tubing 39 adjacent to the seal 19 but spaced slightly from itto provide for unsealed bands 14 and 15.

After being severed by the cutting wheel 47, the sections of tubing passover the plate and between cooperating draw rolls 5? and 51 which aredriven in given feed d'rections by means not shown and are supported byupright members 52 at either end. Roll is preferably provided with anumber of equally spaced grooves 53 about the circumference thereof,each having a fiat bottom which may be an inch or two wide. Plate 43 isprovided with an equal number of tongue like projections 48A whichextend into grooves 53 between rolls 5th and This provision assures thatthe plastic tube will be guided between rolls 5%) and 51.

Downstream from lOlls 5t) and 51 are a pair of hollow bars 6 and 6. .vsupported from uprights 52. A cooling medium such as water is passedthrough the center thereof to maintain the bars at an even temperature.The lower bar 61 is fixed to upright member 62 at either end and theupper bar 66 is free to move up and down in suitable guides. Up and downmotion is imparted to bar 63 by means of pneumatic cylinder 62. Securedto one side of each bar are strips of asbestos 63 and 64 to protect thebars from heat produced by the gas burner 65 hereinbelow described.

Attached to the upper bar by a metal brace 66 is a burner whichcomprises a square hollow tube 67 having a beveled corner 68 which isprovided with a plurality of small holes 69 drilled through the wallthereof. A combustible gas such as butane, propane or natural gas issupplied to the burner 65 through tube 70. A pilot flame 71 is providedby a small burner 72. Gas is supplied to the burner 72 by pipe 73.

When the bars 66 and 61 are brought together on either side of the endof the plastic tube 39 close to seal joint 10, gas is introduced toburner 65 through tube 70. The fiame or" the burner is lighted by pilotflame 71 and the edge of the plastic tube 30 extending beyond the end ofthe seal is melted into the greater thickness thereof to form a joint17.

The entire station used to form the finished joint comprising uprightmembers 52 and associated rolls 50 and 51, clamps 60 and 61, etc. ismovably mounted on frame 29 by a number of small wheels 75. The positionof the entire station with respect to the frame can be changed byadjusting set screw 76 so that the apparatus may be used for making bagsof various lengths.

During operation, both pairs of draw rolls are rotated a predeterminednumber of turns and then stopped to advance any desired length oftubing. Heat sealing bar 37, clamp 64 gas burner 65 and the cutter 46are then actuated simultaneously. The time and pressure required forsealing are controlled by adjustable commercially available regulatorsas is well known inthe art. When the time required for sealing haselapsed, the sealing bars and clamps are separated and the gas is turnedoil. The first seal 10 and separation of the tubing for one bag willthen be completed at the first station and the finished joint foranother bag will be completed at the second station. Another section oftubing is advanced by again actuating both sets of rolls to beginanother operating cycle, preliminary seal 10 made in the first stationbeing formed into a finished joint at the second station. If the secondsealing stage is operated alone without the first stage, that is, bysealing aligned edges which have undergone no previous sealing, theresulting joint is unsatisfactory in strength and usually exhibitsoccasional gaps or openings as a consequence of the two sheets meltingaway from one another when exposed to heat.

It is apparent that many modifications and variations of this inventiona hereinbefore set forth may be made Without departing from the spiritand scope thereof. The specific embodiments described are given by Wayof example only and the invention is limited only by the terms of theappended claims.

I claim:

1. A method of joining together the edges of thermoplastic resinoussheet material which comprises first sealing together superimposedsheets by subjecting to heat and pressure over a narrow elongated areaparallel to and spaced from the edges of the superimposed sheets toleave a narrow area of unsealed material adjacent the edges, and thenmelting said area of unsealed material between the sealed area and theedges of the sheets by application of heat to produce a thickened beadintegral with said sealed area and of increasing thickness from saidsealed area outward toward the free edges of the sheets.

2. A method according to claim 1 further characterized in that saidthermoplastic resinous material is a polyolefin.

3. A method according to claim 2 further characterized in that saidpolyolefin is tubular polyethylene.

4. A method according to claim 3 funther characterized in that saidpolyethylene has a thickness between about 5 and mils.

5. A method according to claim 1 further characterized in that saidsheet material adjacent to the sealed area but spaced away from the freeedges of the sheets is artificially cooled during application of heat tothe unsealed area between the sealed area and edges of the sheets.

6. A method according to claim 1 further characterized in that said heatfor melting is applied by a plurality of gas jet flames.

7. A method of joining together the edges of thermoplastic resinoussheet material comprising first sealing superimposed layers ofthermoplastic resinous sheet material together by heat and pressurealong a narrow band parallel to and spaced inwardly from the edges ofsaid sheets, the edges of the said sheet material projecting outwardlyfrom said band comprising extensions of unsealed sheet material, andthereafter subjecting the seal to heat above the melting temperaturethereof for a period of time less than that required to bring the entireexposed portion of said sheet material to the melting temperaturethereof whereby said extensions are caused to melt into the greaterthickness of said seal, the cross-sectional thickness of said seal isincreased and the material of said extensions is formed into an integralbulbous mass along the free edge of said seal.

8. Apparatus for joining together the edges of thermoplastic resinoussheet material comprising means for intermittently advancing a pluralityof superimposed layers of resinous sheet material; a first sealingstation in the path of said advancing means, said station including apair of sealing bars extending across the path of said advancing means,heating elements for heating said sealing bars, means for bringing saidbars together on opposite sides of the advancing sheet material to applypressure thereto; and a second sealing station in the path of saidadvancing means, said second station including a pair of clamping barsextending across the path of said advancing means, means for coolingsaid bars, means for bringing said bars together on opposite sides ofthe advancing sheet material to grip the same, and intermittent heatingmeans extending across the path of said advancing means immediatelyadjacent one of said clamping bars to apply heat to one edge of saidresinous sheet material extending out from said cooled clamping bars,said sealing bars, clamping bars and intermittent heating means beingoperable together between operations of said advancing means.

9. Apparatus according to claim 8 further characterized in that saidintermittent heating means includes a tubular conduit extending acrossthe path of said advancing means, a plurality of jet openings in saidtubular conduit directed diagonally into the path of said advancingmeans, means for intermittently supplying a combustible gas to saidconduit means and pilot means for igniting said gas.

10. Apparatus according to claim 8 further characterized by theprovision of means adapted to hold a substantial supply of said resinoussheet material to be joined together in the form of a rolled web ofsubstantial length, said supply means being disposed in the path of saidadvancing means, and means associated With said first sealing station tosever said web of resinous material into shorter lengths.

References Cited in the file of this patent UNITED STATES PATENTS1,825,308 Dorogi et al Sept. 29, 1931 2,488,212 Lloyd Nov. 15, 19492,679,469 Bedfo-rd May 25, 1954 2,682,910 Piazze July 6, 1954 2,744,844Wood May 8, 1956 2,786,511 Reid Mar. 26, 1957 2,979,113 Stageberg Apr.11, 1961 2,987,107 Sylvester June 6, 1961 2,994,361 Gable Aug. 1, 1961FOREIGN PATENTS 544,710 Canada Aug. 13, 1957

1. A METHOD OF JOINING TOGETHER THE EDGES OF THERMOPLASTIC RESINOUSSHEET MATERIAL WHICH COMPRISES FIRST SEALING TOGETHER SUPERIMPOSEDSHEETS BY SUBJECTING TO HEAT AND PRESSURE OVER A NARROW ELONGATED AREAPARALLEL TO AND SPACED FROM THE EDGES OF THE SUPERIMPOSED SHEETS TOLEAVE A NARROW AREA OF UNSEALED MATERIAL ADJACENT THE EDGES, AND THENMELTING SAID AREA OF UNSEALED MATERIAL BETWEEN THE SEALED AREA AND THEEDGES OF THE SHEETS BY APPLICATION OF HEAT TO PRODUCE A THICKENED BEADINTEGRAL WITH SAID SEALED AREA AND OF INCREASING THICKNESS FROM SAIDSEALED AREA OUTWARD THE FREE EDGES OF THE SHEETS.